This invention relates to liquid ink printing, and is more particularly concerned with a multicolor liquid ink printer for producing unmottled, high quality multicolor liquid ink images on plain paper.
In existing thermal ink jet printing, the printhead typically comprises one or more ink ejectors, such as disclosed in U.S. Pat. No. 4,463,359, each ejector including a channel communicating with an ink supply chamber, or manifold, at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet.
The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a print sheet, such as a sheet of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Other types of ink-jet printing, such as piezoelectric, continuous-stream, or acoustic, are also known, and are also applicable to the present invention.
In a single-color ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead moves across a print sheet numerous times in swaths, much like a typewriter; alternatively, a printhead which consists of an array of ejectors and extends the full width of the print sheet may be passed once down the print sheet to give full-page images, in what is known as a "full-width array" (FWA) printer. When the printhead and the print sheet are moved relative to each other, imagewise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print sheet.
With ink-jet printing, it is also possible to create multicolor images on a print sheet. This type of printing may be used for full-color images, such as to reproduce a color photograph, or can be employed for "highlight" color, in which colored additions are made to a main portion of the image or text, which is typically black. In either case, the most common technique for color ink jet printing has been to sequentially image two or more colors, in separate printing steps, onto the single print sheet. This superimposition can be carried out in any number of ways. To take the example of a full-width apparatus printing black and one highlight color, an apparatus may print out the entire black portion of the desired highlight image on the sheet, and then recirculate the print sheet once again to image the highlight color portion of the image onto the same sheet from another printhead loaded with the colored ink; such a system has a serious disadvantage in the production of accurate registration of the composed images.
Alternately, two printheads may be positioned very close to each other, and render the two portions of the image onto the print sheet almost simultaneously, although two different areas of the print sheet will be printed upon by the different printheads at the same time or with a small time lag. For a full-color process image, four types of ink (yellow, magenta, cyan, and black) are emitted from four separate printheads during printing as the print sheet is moved relative to them.
In any ink-jet printing apparatus, but particularly in color-printing applications, one key concern is the rapid and efficient drying of the ink which has been placed on the print sheet by the printheads. If wet ink is allowed to remain on the print sheet for an appreciable length of time, the image is likely to smear as the print sheet continues on its path through the apparatus. In color ink jet printing situations, another important problem related to ink drying is known as "intercolor bleed." This is a bleeding of one color portion of the image into another portion of the neighboring image of different color. This becomes most apparent when a black image is imaged immediately adjacent to an area printed with a color such as cyan, magenta, or yellow. In such a case, the black ink will be seen to bleed into the color area (e.g., cyan, magenta, and yellow) to create a conspicuous print defect. If a composite color is made in the color area (e.g. by combining cyan and magenta to make a shade of blue), the problem will be particularly acute because of the large amount of liquid on the sheet surface. The lighter colored ink will bleed into the black portions of the image as well, but bleeding in this direction will not be as noticeable.
Heat and delay printing has been identified as a key technique for achieving high quality color liquid ink printing on plain paper. This is usually demonstrated by printing a slow dry black which yields low MFLEN values (sharp edges) onto a heated paper. Then after approximately one second delay, a fast dry color ink is printed. While printing on the heated paper may not improve the black MFLEN, it will significantly prevent the black ink from bleeding into the color inks (intercolor bleed).
Although the fast dry color printing has some feathering, the black printing creates the impression of a sharp printed result. When printing solid areas with a slow dry black ink on a heated substrate, a mottled print often results. This can be reduced by checkerboard printing techniques which allow small printed areas to dry prior to spreading. Checkerboard printing also minimizes print curl and if the printhead is shifted lengthwise between the two printing cycles, printhead signature and/or defects are also minimized.
To reduce the print defects caused by inefficient drying of ink on the substrate and by intercolor bleed, it is however well known to use special but expensive coated sheets or paper as opposed to plain or uncoated paper which ordinarily result in very poor quality images. Additionally, "quick penetrating" inks or special printing techniques such as checkerboard printing disclosed for example in U.S. Pat. No. 4,748,453 issued to Lin et al, and heat and delay printing disclosed for example in U.S. Pat. No. 5,570,118 issued to Rezanka et al., must be used. Attempts to use such quick-penetrating inks to similarly print liquid ink images on plain paper ordinarily result in poor quality images having a defect such as "mottle".
There is therefore still a need to provide an ink-jet color printing apparatus having an architecture and including printhead structures and electronic control subsystems for producing unmottled, high quality multicolor liquid images on plain paper.